1. Field of the Invention
The present invention relates to semiconductor wafer handling and processing equipment, and in particular, to a method and apparatus for calibration of a wafer handling robot relative to a station in a semiconductor workpiece tool.
2. Description of the Related Art
The introduction of workpiece handling robots into the semiconductor wafer fabrication process represented a significant advance in automation over manual and early transfer equipment for moving wafers between various stations at a workpiece tool, such as process tools and/or workpiece storage and handling locations. It is an important feature of conventional workpiece handling robots to be able to quickly and precisely access a workpiece from a first position, deliver it to a new location having different X, Y and Z coordinates in cartesian space, and set it down without risk of damage to the workpiece.
In order to accomplish this, a typical robot includes a shaft mounted in a base for translation along a vertical axis. A first arm, or link, is rotatably mounted to an upper end of the shaft, and a second arm, or link, is rotationally mounted to the opposite end of the first arm. The workpiece handling robot further includes an end effector pivotally attached to the second arm for supporting the workpiece. Various motors are further provided, conventionally mounted in the base, for translating the shaft, and for rotating the first and second arms such that the end effector may be controllably maneuvered in three-dimensional space.
To ensure that the end effector of the workpiece handling robot is precisely located during handling of workpieces, the robot must be calibrated relative to a workstation to and from which the robot transfers the workpieces. Each time the workstation is moved, as for example when it is replaced or repaired, the calibration process must be performed to ensure proper positioning of the robot to the workstation. Often a single robot operates with a plurality of work stations within a workpiece tool. In this instance, not only must the robot be calibrated to work with each of the stations, but any time one of the stations is moved, for reasons explained hereinafter, not only must the robot be re-calibrated to operate with the moved station, but it often must be re-calibrated to operate with each of the workstations serviced by that robot.
The calibration, or xe2x80x9cteachingxe2x80x9d, of a robot allows the robot to commit to stored memory the location of the station(s) relative to the robot. In order to calibrate (or re-calibrate) the robot to a particular station, a rough approximation of the correct positioning between the robot and station is established by the operator. Thereafter, the end effector is moved to the position on the workstation which the operator estimates is the proper position of the end effector to drop off and acquire workpieces to and from the station. Typically, the operator visually aligns an end effector with the center position of the workpiece supported in a cassette on the station. The operator may use a remote control xe2x80x9cteach pendantxe2x80x9d to position the end effector, or the operator may manually drag the end effector into what the operator estimates to be the proper position. This relative position between the robot and station is then stored in memory.
Conventional calibration procedures have many disadvantages. Visually aligning the end effector with the center of the workpiece has proven to be inaccurate and subjective because it is dependent on the accuracy of each individual operator. Furthermore, the structure of the cassette largely prevents visual inspection of a workpiece therein to determine its center. The operator in most cases only has visual access through a front of the cassette through which the workpieces are transferred. Moreover, in the case where the operator manually drags the arm of the robot, additional forces are placed on the robot which can lead to later difficulties with the functioning of the robot.
In addition, the calibration procedures have not been repeatable because there are no standard procedures for calibration of a workpiece handling robot to a station. One problem has been that the calibration procedure used by one operator to calibrate a workpiece handling robot to a station may be different than the calibration procedure used by a second operator calibrating the same robot to the same station. For example, a first operator may align a first point on the robot with the center of the workpiece, while a second operator uses a second, different point. This could result in faulty workpiece transfer to an existing station, even though the robot is properly calibrated to the new or modified station. Thus, as discussed above, when one station of a plurality of stations serviced by a single robot is added or otherwise moved, not only is it necessary to calibrate that particular station, but in many situations, each of the other stations must also be re-calibrated to ensure that each of the stations serviced by the robot are calibrated in the same way and off of the same reference points.
A still further problem has been that the calibration procedure used by the operator at one station may be different than the calibration procedure used by the same operator at a second station. As a result of these inconsistencies, one station may significantly out perform another station or cause damage to a workpiece. Since there is no repeatable and consistent calibration procedure, the reason for a particular performance at a station or for damage caused to a workpiece is not determinable.
Moreover, the calibration procedures have not been quantifiable because there is no way to compare the quality of the calibration procedure used by one operator to the quality of the calibration procedure used by a second operator. Similarly, there is no way to compare the quality of the calibration procedure used at one workstation to the quality of the calibration procedure used at a second workstation.
In an effort to deal with this problem, a known calibration method includes a sensor which is installed at every station in a workpiece tool. The sensor may be mounted at any position on the station which is reachable by the end effector so long as the location of the sensor does not interfere with the processing operations at the station. The calibration procedure is completed by having the robot seek the sensor in order to identify the location of the workpiece. This method has the advantage of eliminating errors due to operator inaccuracies. However, if there are a plurality of stations, each sensor must be mounted and calibrated with respect to the robot. For example, if there are twelve stations, then twelve sensors must be mounted and calibrated with respect to the wafer handling robot, or one sensor must be moved around manually to each of the twelve stations. Therefore, more sensors means the calibration system will be more expensive and time consuming.
In one known system, a support structure including a sensor is mounted directly to a work station in a know position relative to the work station. The robot is then coarsely aligned over the station, and is moved around so that the sensor can identify one or more outer edges of the robot. Once the outer edges of the robot have been identified, the position of the robot relative to the sensor and station can be determined and stored. A drawback to this type of system is that the sensor structure must either be mounted to each station, or a single sensor structure must be moved between each station, in order to align the robot to each station in the tool. Additionally, the sensor and control system are configured to identify the edges of an end effector of a single, known configuration. However, there is not a single uniform shape to end effectors, and thus the sensor structure has limited use.
It is therefore an advantage of the present invention to provide a robot calibration system which allows for determination of a position and orientation of a workpiece handling robot relative to workstations in a workpiece tool.
It is another advantage of the present invention to provide a robot calibration system which is time efficient and allows for easy calibration of the workpiece handling robot relative to a station.
It is still a further advantage of the present invention to provide a robot calibration system which allows for calibration of the workpiece handling robot relative to a plurality of stations in a single calibration session.
It is another advantage of the present invention to provide a robot calibration system which is inexpensive and requires only one sensor to detect relative position of robot and station in a horizontal plane.
It is a further advantage of the present invention to provide a robot calibration system which is capable of precise positional accuracy and repeatability.
It is yet a further advantage of the present invention to provide a robot calibration system which includes components that are easily accessible and visible to the operator.
It is yet another advantage of the present invention to provide a robot calibration system which is efficient by minimizing operator intervention during the calibration process.
It is still a further advantage of the present invention to provide a robot calibration system which utilizes a target which may fit into any station without interference.
It is another advantage of the present invention to provide a robot calibration system which includes a sensor on the workpiece handling robot that is easily and quickly attachable and detachable, or which may alternatively be built integrally into the end effector.
It is yet a further advantage of the present invention to provide a robot calibration system which is universally applicable to a majority of substrate handling tasks utilizing a robot.
It is still a further advantage of the present invention to provide a robot calibration system with a simple design and low number of parts to facilitate easy configuration.
These and other advantages are provided by the present invention, which in the preferred embodiment relates to a robot calibration system for determining a precise position of a workpiece handling robot relative to a workstation in a workpiece tool. In the preferred embodiment, the workpiece handling robot includes a sensor, preferably a retro-reflective optical sensor, located near an end of a robot end effector. The sensor is removably mounted on the end effector in between two fingers and a base portion which form a U-shaped wafer support platform. The sensitivity of the sensor may be set to detect a minor change in reflected light.
The robot calibration system further includes a target including a reflectance pattern on its surface, which target is located by an operator on the station. The target may be the same size or slightly larger than a typical workpiece loaded on the station, and is located on the station in the same position as workpieces that are to be transferred to and from the station by the robot. It is also contemplated that the target be different shapes and sizes than the typical workpieces used on the stations, provided that it sits in a fixed position on the station. By movement of the end effector around portions of the target, the sensor in combination with the robot control system is able to identify the center of the target. Once the center of the target on the station has been identified, the control system knows the spacial relation between the robot and the station, and in particular the position of the workpieces at the station. The identified center position of the target relative to the robot is then stored in the control system memory. Thereafter, the control system may return the end effector to that position to acquire wafers from or present wafers to the station.
The reflectance pattern on the target is on at least one side of the target and preferably on a bottom side of the target. While various reflectance patterns are contemplated, a preferred reflectance pattern on the target comprises four quadrants formed by alternating black areas and white areas. The four quadrants are bordered by two lines which are perpendicular and intersect. The intersection of the two lines represents the center of the target.
The target may be formed of any of various durable materials such as carbon fibre. Where carbon fibre is used, a white reflectance pattern may be applied to the black fibre. Alternatively, the target can be formed of a shiny material, with a black pattern applied to the surface. In a further alternative, both the white and black patterns can be applied to the target. Regardless of how the reflectance pattern is applied, the retro-reflective sensor is able to sense the transition between the black areas and the white areas on the target.
In operation, when it is desired to orient a robot to a station for workpiece transfer between the station and robot, the workpiece handling robot is placed in an approximate orientation to the station, which includes a support surface for the target to be supported thereon. The target may be placed in the bottommost slot in a workpiece cassette which is then placed on the support surface. Alternatively, the support surface may include structures for supporting the target at the same x, y, and z coordinates as would be a workpiece in the bottommost slot of a workpiece cassette on the support surface.
With the robot initially at its lowermost z-axis position, the arms of the robot move up vertically along the z-axis via the shaft with the end effector and sensor positioned under the target. The end effector further includes a vacuum sensor. When the end effector reaches the target, the vacuum sensor detects proximity between the workpiece and sensor. This position corresponds to the z-axis height of the robot for the end effector to be aligned under the bottommost workpiece in the cassette. The vertical position of the bottommost workpiece is stored in the control system.
The end effector then moves vertically down slightly from the target so that there is a small distance between the end effector and the target. The end effector proceeds to move in a search pattern moving in radial (xe2x80x9crxe2x80x9d) and rotational (xe2x80x9cxcex8xe2x80x9d) directions relative to the central axis of the robot, searching the reflectance pattern of the target for transitions from black areas to white areas and visa-versa on the bottom of the target. It is understood that the end effector can maneuver under the target in alternative patterns, such as for example a circular pattern or a square pattern. The parameters determining the search patterns are stored in the control system, allowing the control system to control the maneuvering of the end effector through the search pattern automatically without operator intervention. An indicator, such as an LED light, on an arm of the robot indicates the transition from black areas to white areas and visa-versa. In one embodiment, the sensor is illuminated when located under the black area, and is not illuminated when located under the white area.
The end effector may take one entire pass under the target such that a complete search pattern is formed in order to determine the transition points between the quadrants. The location of four transition points are used by the control system to determine the location of the two lines and the coordinate where the two lines intersect. Using an algorithmic equation, the control system translates the location of the transition points, which are defined in a polar coordinate system (r, xcex8), into coordinates in a Cartesian coordinate system (x, y). As two points define a line, the control system uses the first and third transition points detected during the search pattern to determine the orientation of the first line, and then uses the second and fourth transition points to determine the orientation of the second line. The control system then determines the Cartesian coordinates of the point of intersection of the two lines by finding the single set of coordinates lying on both lines. The intersection of the two lines defines the center of the target. This point is then stored in the control system memory, and the end effector may thereafter be repeatedly located at this point to present and acquire workpieces to and from the workstation.